Thermosetting polyesters having long been used in surface coatings. Hydroxyl and carboxyl groups are most frequently utilised to impart reactive functionality to these resins. In the area of powder coatings, hydroxyl-functional polyesters are usually cured using alkoxymelamine, anhydride or blocked isocyanate crosslinkers. Typical of the hydroxylated polyesters known for use in coating formulations are those derived from various combinations of terephthalic acid, neopentyl glycol, cyclohexanedimethanol, and polyols such as trimethylolpropane. Such polyesters are generally amorphous and have relatively high melt viscosities at fusion temperatures. These resins, at a typical fusion temperature of 160.degree. C., have melt viscosities in excess of about 6500 cps, and often in excess of 10,000 cps.
The high melt viscosity tends to limit the flow of the molten coating and hence adversely affects the smoothness and gloss of the finished coating. Cure speed of these resins depends upon the type of crosslinking agent used. They are all designed for cure with blocked isocyanates but none are recommended for use at cure schedules less than 160.degree. C. for 35 minutes. Below these recommended temperatures, the coatings have a poor appearance and poor physical properties are obtained.
Patents of interest include U.S. Pat. No. 4,094,721 which discloses copolyesters of terephthalic acid, 1,4-butanediol and 1,6-hexanediol. These copolyesters, however, have a relatively high molecular weight and a relatively high melt viscosity, which satisfies the requirements for the intended purpose, i.e., a fusion adhesive. The high melt viscosity tends to limit the flow of the molten coating and hence adversely affects the smoothness and gloss of the finished coating. Cure speed of these resins depends upon the type of crosslinking agent used.